Single crystals of brittle materials



Unite This invention relates to brittle materials and in particular is concerned with the production of single crystals of brittle materials.

There are several known methods for growing single crystals from polycrystalline base material. For example, single crystals have been obtained by straining or stretching polycrystalline material a critical amount and then slowly moving the sheet into the hot zone of a high temperature gradient furnace. Single crystals have also been obtained by solidifying material from a melt, using a seed crystal, and growing the crystal in a temperature gradient.

The strain-anneal method has not, heretofore, been useful for growing single crystals of brittle materials because such materials, when stretched, fracture. Growth from a melt is not always a convenient method for very high melting point materials or where the material involved undergoes a phase transformation. Another drawback of the melt procedure is that segregation usually occurs in multi-component melts and the resulting product is of non-uniform composition. Moreover, whenever it is necessary to use a crucible to contain a molten material, the melt may become contaminated by impurities such as dissolved gases or materials from the crucible itself, thereby leading to degradation of the molten material and products obtained therefrom. Moreover, since the brittle materials may not be workable, single crystals may grow only in some undesired shape rather than in a predetermined shape as in our invention.

It is a major object of the present invention to provide a new method whereby a single crystal of a brittle material may be grown in the solid state.

It is another object of this invention to provide a method for the production of single crystals of brittle materials that is simple; that maybe practiced with known techniques and equipment; and that is applicable to single metals and to alloys.

These and other objects are attained in accordance with our invention by heating a brittle material to an elevated temperature at which the material evidences sufiicient ductility to permit stretching, without fracture or appreciable recovery, thereby inducing a strain in the material. The strained material is then annealed under conditions of a high temperature-gradient to grow the single crystal. In this manner, we are able to produce single crystals of metals and alloys, single crystals of which have been unknown heretofore or for which no satisfactory method of production has been known. Moreover, this method of growing single crystals may frequently be less expensive then growing crystals from the melt.

Our invention accordingly involves heating a brittle material to a temperature at which it evidences sufilcient ductility to permit straining Without fracturing. The temperature may vary within a range from the minimum temperature at which sufiicient ductility exists that permits straining, within the range of about two to six percent, to the recovery temperature for the material involved. Consequently, it should be apparent that no absolute numerical temperature range can be set forth because the range depends on the material being used and in many instances will be quite different for different materials. By way of example, the temperatures used for aluminum-iron alloys containing about 14 to 18 weight rates Patent Patented Mar. 27, 1952 2 percent of aluminum fall within the range of about 200 C. to 500 C.

To determine a suitable temperature for the straining step for any given material constitutes a routine experiment for a laboratory technician and involves straining samples of the material under tension at different temperatures and observing the temperatures at which sufficient stretching occurs without fracture of the sample thereby establishing the minimum temperature that can be used. The maximum temperature, i.e. the temperature below which appreciable recovery does not occur, can also be established routinely as by heating strained samples at different temperatures and observing the decrease in electrical resistance that accompanies recovery. By the term recovery temperature we mean to indicate that ternperature at which residual stresses in a material are relieved, within a short period of, say, about 30 minutes, without encountering a change in gross microstructure or grain size. As is apparent to the artisan, the recovery temperature of any material never exceeds the recrystallization temperature and generally is lower.

After the material has been strained the critical amount, i.e., within about 2 to 6 percent, it is subjected to a steep temperature-gradient to grow the single crystal. This can be accomplished by advancing the sample through a single crystal furnace as rapidly as the growth of the crystal permits. A single crystal furnace is a heating device which is constructed to permit the heating of a small area of a sample to the temperature at which the atoms have suflicient thermal agitation to be able to transfer themselves from the strained lattice onto a new unstrained crystal. The essential aspect of the heating device accordingly is the ability to bring, substantially simultaneously, all the atoms that are close to the new crystal to a high enough temperature so that they join with that new crystal rather than take part in the nucleation of a different new crystal. Hence any furnace that has a heating device that can produce a high temperature in a localized area normally will be satifactory for use in practicing our invention. A resistance winding generally is entirely adequate for these purposes. It is readily apparent that the temperature to be used in the high temperature zone of the furnace is determined by the material being treated; however, that temperature must exceed the recrystallization temperature. For aluminum-iron alloys a temperature in the hot zone of at least about 1000 C. is used. The material is fed to the high temperature zone from an area in which no intentional heating occurs, or from an area that is provided with cooling means, whereby the gradient is established.

This invention concerns the preparation of single crystals of brittle materials. A material is brittle, as that term is used in this specification, when it fractures upon attempting to introduce a critical strain therein about 2 to 6 percent, as by rolling or tensile straining, at room temperature. Such materials include, by way of example aluminum-iron alloys containing at least 14 percent by weight of aluminum. The brittle materials may contain alloying constituents, other than as indicated, and impurities in amounts usual therefor as long as they do not deleteriously interfere with obtaining single crystals in the described manner.

The brittle material may be used in this invention in a variety of shapes such, for example, as strip and wire. These may be obtained in any manner desired. However the material should be free of induced stresses and should have a uniform and fine grain structure. Consequently, it is desirable to prepare the base material by annealing at a temperature above the recrystallization temperature for a time sufficient to complete stress-relief and recrystallization. Where necessary, this is done in an atmosphere that will not deleteriously attect the base material or introduce undesirable impurities. Where the material must be deformed to provide, by way of example, sheet from which strips are to be cut, suitably a stress-relief anneal is used just prior to initiation of the steps of this invention. Where strips or special shapes are used, they may be cut from a sheet by a stress-free procedure, e.g. acid or sparkcutting or the like, thereby avoiding a need for stressrelief on these shapes as such. The shapes that are used generally are 5 to 50 or more mils in thickness and about an eighth to one inch or more in width.

The invention will be discribed further in conjunction with the following example. It should be understood that the details disclosed are not to be construed as limiting on the invention.

An ingot of an alloy, of 16 weight percent aluminumiron was prepared by melting electrolytic iron and high purity aluminum and then vacuum casting the melt. Material of this composition is brittle; that is, it cannot be cold rolled or stretched at room temperature without fracture. The hot top of the ingot was removed and the ingot was then hot rolled at about 1000 C. to a thickness of one-half inch. Thereupon the resulting plate was Warm rolled at a temperature ranging between 525 to 575 C. to 0.029 inch. Strips were cut from the sheet and were annealed at 1000 C. in an inert atmosphere for one hour. After furnace cooling the strips to room temperature, they were reheated to 400 C. and then were stretched in a tensile machine to produce an elongation of 4 percent. The strained strips were then advanced into the hot zone of a single crystal furnace having its high temperature area maintained above 1100 C. and single crystals were grown.

Thus, single crystals of the brittle 16 percent aluminumiron alloy were obtained by a strain-anneal process, a result hitherto impossible. Moreover, this result was achieved without producing segregation in the aluminumiron, a characteristic of single crystals of this material that we observed on growing such crystals from a melt. The crystals were obtained in sheet form, thereby demonstrating that the invention is not limited to obtaining bar shapes as are other processes.

From what has been disclosed it will be apparent that we have provided an efiicient method of producing single crystals of brittle materials. While the invention has been exemplified as it applies to an aluminum-iron alloy, it will be apparent that the invention can be practiced with other brittle materials albeit with operating conditions other than those shown for aluminum-iron.

The single crystals prepared in accordance with our invention can be used for purposes similar to the uses of the commercial magnetic'silicon-iron and nickel-iron alloys. Typical applications include use in pulse or current transformers, relays, meters and various A.-C. in struments.

In accordance with the provisions of the patent statutes we have explained the principle of our invention and have described what we now believe to represent its best embodiment. However, we desire to have it understood that the invention may be practiced otherwise than as specifically described.

We claim as our invention:

1. The method of preparing a single crystal of a brittle material selected from the group consisting of metals and alloys which will fracture when strained from 2% to 6% at room temperature but are ductile at elevated temperatures when strained this amount, the steps comprising heating a body of the brittle material to the elevated temperature but below the recovery temperature of the material, thereupon straining the heated body while at the elevated temperature to stretch the material within the range of about 2% to 6%, and then annealing the body of the strained material by heating an initial small portion of the body above the recrystallization temperature of the material so as to maintain a high temperature gradient between the small portion and the remainder of the body and progressively subjecting the remainder of the body to the elevated temperature as single crystal growth progresses from the initial small portion.

2. A method of preparing a single crystal of a brittle alloy consisting essentially of about 14 to 18 percent by weight of aluminum and the balance substantial all iron and incidental impurities which comprises heating such a material to a temperature within the range of about 200 to 500 C. straining said material at said temperature to stretch it critically Within the range of about 2 to 6 percent, and thereafter annealing the strained material to grow a Single crystal thereof by moving it through a zone of high temperature gradient.

General Electric Review, vol. XXV, No. 5, page 315. Metal Progress, December 3, pages 71 to 75, especially, Method 2, page 72. 

2. A METHOD OF PREPARING A SINGLE CRYSTAL OF A BRITTLE ALLOY CONSISTING ESSENTIALLY OF ABOUT 14 TO 18 PERCENT BY WEIGHT OF ALUMINUM AND THE BALANCE SUBSTANTIAL ALL IRON AND INCIDENTAL IMPURITIES WHICH COMPRISES HEATING SUCH A MATERIAL TO A TEMPERATURE WITHIN THE RANGE OF ABOUT 200* TO 500* C. STRAINING SAID MATERIAL AT SAID TEMPERATURE TO STRETCH IT CRITICALLY WITHIN THE RANGE OF ABOUT 2 TO 6 PERCENT, AND THEREAFTER ANNEALING THE STRAINED MATERIAL TO GROW A SINGLE CRYSTAL THEREOF BY MOVING IT THROUGH A ZONE OF HIGH TEMPERATURE GRADIENT. 